Current automotive vehicle manufacturing operations include, for example, the joining of two sheet metal layers by spot welding. Vehicle body panels such as doors, hoods, deck lids and liftgates are often assembled by joining inner and outer panels stamped from sheet metal of suitable metal alloys. Ferrous or aluminum alloys are often used. The thickness of each sheet metal layer may vary from less than one millimeter to more than four millimeters. Electrical resistance spot welding is often used to join such inner and outer panels or other metal parts.
In the case of sheet metal body components, flats or flanges of two or three components are placed together and then a series of spot welds penetrating from the top sheet layer into the bottom layer are made to securely attach the panels. Welding practices have been developed for such spot welding operations. Good welding practices are particularly critical in joining aluminum sheet alloys because of the high electrical and thermal conductivity of the material and the omnipresent oxide coating on the surface. Similar welding challenges arise in the welding of other light metal workpieces such as parts made of magnesium alloys. The spot welding operation is accomplished by assembling the parts in a suitable fixture and pressing welding electrodes against opposite sides of the overlying or touching parts at the intended weld location. The welding electrodes usually provide both clamping force and current commutation for the weld.
Copper or copper alloy welding electrodes are often used in welding aluminum alloy workpieces. U.S. Pat. No. 6,861,609, titled Welding Electrode for Aluminum Sheets and assigned to the assignee of this invention, illustrates some such welding electrodes.
As illustrated in the '609 patent, the electrodes are often round cylinders with a welding face at one end shaped to engage the workpieces. The welding electrodes are part of a welding apparatus including a welding head or gun that can be moved and actuated to press two aligned and opposing electrodes against the assembled workpieces. The apparatus then delivers a momentary welding current to the electrodes to affect the weld. Workpiece metal layers between the electrodes are momentarily melted by electrical resistance heating to form a weld nugget joining the layers. The clamping force, the value of the welding current (often single phase alternating current, 60 Hz, or rectified direct current) and current duration (several cycles of the 60 cycle current) are also specified for the electrodes to be used and the welding task.
In vehicle manufacturing or other industrial process, each welding gun is typically used to make a rapid succession of welds, for example, around the periphery of two or more overlying panels. The high electrical and thermal conductivity in combination with the insulating nature of the naturally-formed surface oxides of aluminum alloys (or magnesium alloys) makes them difficult to weld using spot welding practices previously developed for steel alloys. In the case of light metal alloys, the spot welding process is sensitive to a large number of variables beyond the normal welding parameters of electrode configuration, electrode force, weld time, and weld current. These other variables include sheet surface oxidation, sheet surface cleanliness, sheet surface topography as well as process variations such as alignment of the electrodes to the sheet, location of electrodes relative to the sheet edge and part radius, metal fit up, gun stiffness, alignment of electrodes on the gun, electrode surface roughness, and wear of the electrode surface.
The welding faces of some electrodes are roughened by blasting with small steel or sand particles or abrasion with a coarse abrasive paper as illustrated in the '609 patent. The roughened surface is characterized by randomly distributed craters with peak to valley dimensions, for example, in the range of 5 to 30 micrometers and with substantially the same range of peak to peak spacing. This texture permits the electrode face to penetrate an oxide film on the workpiece surface to reduce electrode resistance (and overheating) at the contact surface of the electrode and part. But, whether textured or not, the tips or welding faces of the electrodes may be altered by erosion or by adhesion and buildup of workpiece material after several welds. Welding operations must then temporarily cease while the electrode faces are cleaned, or re-shaped, or re-dressed. The redressing of grit blasted electrode faces, for example, can require many tens of seconds of off-line processing.
There is a need to provide a resistance welding electrode with a contoured welding face that improves electrical contact with a workpiece surface and the reliability of resistance welding, and decreases the time required for re-dressing of the welding face during welding operations. Such an electrode would be useful in many welding applications and would be particularly useful in welding light metal alloy workpieces with their oxide surface films.